Wetland Loss Patterns and Inundation-Productivity Relationships Prognosticate Widespread Salt Marsh Loss for Southern New England

Abstract

Tidal salt marsh is a key defense against, yet is especially vulnerable to, the effects of accelerated sea level rise. To determine whether salt marshes in southern New England will be stable given increasing inundation over the coming decades, we examined current loss patterns, inundation-productivity feedbacks, and sustaining processes. A multi-decadal analysis of salt marsh aerial extent using historic imagery and maps revealed that salt marsh vegetation loss is both widespread, and accelerating, with vegetation loss rates over the past four decades summing to 17.3%. Seaward retreat of the marsh edge, widening and headward expansion of tidal channel networks, loss of marsh islands, and the development and enlargement of interior depressions found on the marsh platform contributed to vegetation loss. Inundation due to sea level rise is strongly suggested as a primary driver: vegetation loss rates were significantly negatively correlated with marsh elevation (=0.96; =0.0038), with marshes situated below mean high water (MHW) experiencing greater declines than marshes sitting well above MHW. Growth experiments with, the Atlantic salt marsh ecosystem dominant, across a range of elevations and inundation regimes further established that greater inundation decreases belowground biomass production of and thus negatively impacts organic matter accumulation. These results suggest that southern New England salt marshes are already experiencing deterioration and fragmentation in response to sea level rise, and may not be stable as tidal flooding increases in the future.

Flooding Alters Plant‐Mediated Carbon Cycling Independently of Elevated Atmospheric CO2 Concentrations

“…Our aboveground production values compare well to studies on S. alterniflora on the subtropical Gulf Coast (Kirwan et al, ; Stagg, Schoolmaster, Piazza, et al, ). We observed a marginal increase in belowground root production and no response in total belowground production in response to greater average flooding depth; this is in contrast to previous studies that have shown decreases in S. alterniflora belowground biomass in response to increased flooding (Snedden et al, ; Voss et al, ; Watson et al, ). Other marsh species also typically respond strongly to flooding in biomass production above‐ and belowground (e.g., Janousek et al, ; Kirwan & Guntenspergen, ).…”

Lateral Marsh Edge Erosion as a Source of Sediments for Vertical Marsh Accretion

“…Hughes et al () and Wilson et al () previously documented headwater erosion and widening for first‐order tidal creeks in Plum Island Sound estuary. Our results agree with observations of marsh shoreline erosion from other tidal wetland systems as well, for example, marsh losses in Louisiana, >25% marsh area lost since late 1800s (Blum & Roberts, ); southern New England, losing marsh at rate of 0.42% yr −1 for past 30–40 years (Watson et al, ); Choptank River in Maryland, losing marsh at rate of 0.11% yr −1 1939–1980 (Yarbro et al, ); and Rehoboth Bay in Delaware, edge erosion at 14–43 cm yr −1 over a 3‐year period in 1980s (Schwimmer, ).…”

Varying Inundation Regimes Differentially Affect Natural and Sand-Amended Marsh Sediments

“…patens , Distichlis spicata , Juncus gerardii [45,46,47]). Under high inundation conditions and when there is low sediment supply, plants need to accumulate enough belowground carbon and build peat to keep up with the increased flooding [9,31,48]. If plant productivity is reduced due to the waterlogged conditions, the marsh will erode and this process will contribute to further deterioration.…”